1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electron-emitting device.
2. Related Background Art
The cathode ray tube (CRT) has long been utilized as the image forming apparatus for displaying an image utilizing an electron beam.
On the other hand, the flat display device utilizing liquid crystal has recently become popular in place for the CRT, but such display device, being not light-emitting, is associated with a drawback of requiring a back light source, and the development of the display device of light-emitting type has therefore been longed for.
For such light-emitting display device, the plasma display device is being recently commercialized, but, being different from the CRT in the principle of light emission, is somewhat inferior to the CRT in the image contrast and the quality of the generated colors. Also a flat panel image forming apparatus utilizing a planar array of plurality of electron-emitting elements is expected to provide the light emission comparable to that of the CRT, and intensive researches and developments have been conducted in this field. For example the Japanese Patent Application Laid-Open No. 4-163833 discloses a flat panel electron beam image forming apparatus utilizing linear hot cathodes and complex electrode structures contained in a vacuum panel.
Such vacuum panel is generally formed by hermetically adhering a glass rear plate, bearing an electron source consisting of a matrix array of plural electron-emitting devices and wirings for driving such electron source, and a glass face plate bearing an image forming member, with a sealing material across a frame, or, in case the gap between the rear plate and the face plate is small, by directly adhering such rear plate and such face plate with a sealing material.
The sealing material is usually composed of low-melting glass, which requires, for softening, a heating process to a high temperature of about 400° C. In this operation, the face plate and the rear plate, as well as other components such as an atmospheric pressure supporting spacer required for constituting the vacuum panel and an anode terminal to be explained later, are exposed to the high temperature at the same time.
The interior of the panel prepared through the above-described process is rendered vacuum by an evacuating process to obtain the vacuum panel. Then, after a process of electrically connecting an external drive circuit with the lead wires formed on the rear plate, the vacuum panel is incorporated in a casing to complete the image forming apparatus.
In thus formed image forming apparatus utilizing the electron beam, while a voltage of several hundred volts to several ten kilovolts for accelerating the electrons is applied between the two glass plates (rear plate bearing the electron source and face plate bearing the image forming member), an image signal is supplied from an external signal processing circuit and through the lead electrodes of the rear plate to emit electrons in the desired position, whereby the emitted electrons are accelerated by the potential difference between the two glass plates and cause light emission from the image forming member of the face plate, thereby obtaining an image. In case the image forming member is composed of ordinary fluorescent material, the above-mentioned voltage is preferably as high as possible in order to obtain the light of desired colors, desirably at least several kilovolts. Therefore, in order to supply the above-described image forming member with the voltage of several kilovolts, there is required a connection structure designed in consideration of the discharge or high voltage.
Such image forming apparatus is provided, in the image forming member, with an anode lead portion for supplying the image forming member with the high voltage. For example the Japanese Patent Application Laid-Open No. 10-326581 discloses anode terminal structure in which a high voltage supplied from a high voltage generating source is supplied by a high voltage cable to an anode lead portion of the rear plate side, then supplied through a lead wire to a wiring lead from the image forming member formed on the face plate and to such image forming member.
Such image forming apparatus has been associated with the drawbacks that:                1. no place is available for forming the anode lead portion;        2. a single lead portion results in a sloped luminance, leading to unevenness in the image, in case of a large area panel; and        3. a thin image forming apparatus of flat panel type involves danger of discharge because the distance between the image forming member and the electron source along the internal wall of the vacuum envelope becomes short. The discharge, if generated, involves an instantaneously very large current, and a large voltage is applied to the electron-emitting device of the electron source if a part of such current flows into the wirings of the electron source. If such voltage exceeds that applied in the normal operation, there may result deterioration of the electron-emitting characteristics, of eventually destruction of the electron-emitting device. In such case, a part of the image becomes undisplayable, whereby the image quality is severely deteriorated and the image forming apparatus becomes no longer usable.        
For these reasons, there has been desired a large-area electron beam image forming apparatus of high reliability, adapted for forming a thin structure and capable of resolving the above mentioned drawbacks.